Fabrication and Characterization of Flexible Substrates for Use in the Development of Miniaturized Wireless Sensor Network Modules

In this paper we describe the materials-related challenges in applying folded flex packaging technology to wireless sensor networks and propose solutions for implementing miniaturized $5mm$ cube platforms. The focus is to apply thin silicon stacking methods using thin flexible substrate interconnect and in particular to investigate the behavior of the selected materials. Both commercial and in-house polyimide substrates, in the thickness range $25μm$ down to $3μm$ (each with $4μm$ of sputtered copper) were analyzed for appropriate electrical, chemical, and mechanical properties. The characterization highlighted that in flex of thickness below $10μm$⁠, a dramatic decrease in stiffness occurs and the polyimide wrinkles due to stresses generated by the copper sputtering process. An evaluation determined that specific steps, such as polymer support ring formation, could be employed to eliminate impact of wrinkling on the process of developing the $5mm$ cube prototypes.